Composition based on cerium oxide or on cerium and zirconium oxides, in the extruded form, process for the preparation thereof and use thereof as catalyst

ABSTRACT

The present invention relates to a composition based on cerium oxide or on cerium and zirconium oxides, in the extruded form, to a process for the preparation thereof and to the use thereof as catalyst. The process for the preparation of the composition of the invention is characterized in that a cerium hydroxide or oxyhydroxide or cerium and zirconium hydroxides or oxyhydroxides are extruded. The composition of the invention can be used as catalyst or catalyst support, in particular in the treatment of exhaust gases from internal combustion engines, in the process for the dehydrogenation of ethylbenzene to styrene, in the catalysis of methanation or in the treatment of a solution or suspension of organic compounds by oxidation via a wet route.

[0001] The present invention relates to a composition based on ceriumoxide or on cerium and zirconium oxides, in the extruded form, to aprocess for the preparation thereof and to the use thereof as catalyst.

[0002] Compositions based on cerium oxide or on mixtures of cerium oxideand of zirconium oxide are well known. They are used in particular ascatalyst or catalyst support, in particular for the catalysis ofautomobile afterburning. These compositions are generally employed in acoating technique, that is to say by mixing them with a binding oxide,such as alumina or silica, and by depositing the mixture obtained, inthe form of a layer, on a support. However, the binding oxide can resulteither in a rapid deactivation of the compositions or in a loss in theirselectivity.

[0003] On account of this disadvantage, it would be advantageous to beable to obtain these compositions directly in the extruded form. To theknowledge of the Applicant Company, these compositions have never beenable to be obtained until now in this form.

[0004] The object of the invention is thus to provide these compositionsin the form of extrudates.

[0005] The Applicant Company has discovered that the use of a specificstarting material made it possible to solve this problem.

[0006] The invention thus relates to a process for the preparation of acomposition based on cerium oxide or on cerium and zirconium oxideswhich is characterized in that a product is extruded which is based on acerium hydroxide or oxyhydroxide or on cerium and zirconium hydroxidesor oxyhydroxides.

[0007] The invention also covers a composition based on cerium oxide oron cerium and zirconium oxides, characterized in that it is provided inthe extruded form.

[0008] Finally, the invention relates to the use of a composition of theabove type as catalyst or catalyst support, in particular in thetreatment of exhaust gases from internal combustion engines, in theprocess for the dehydrogenation of ethylbenzene to styrene and in thecatalysis of methanation.

[0009] Other characteristics, details and advantages of the inventionwill become even more fully apparent on reading the followingdescription and various concrete but nonlimiting examples intended toillustrate it.

[0010] The product of the invention will first of all be described.

[0011] The essential characteristic of the compositions of the inventionis their shape.

[0012] This is because they are provided in the form of extrudates.Extrudate is understood to mean any object obtained by ejection, underpressure, of a paste through nozzles or dies of chosen shapes. Theobjects thus obtained can exhibit varied shapes. They can, for example,exhibit cylindrical or semi-cylindrical, square or polygonalcross-sections or alternatively cross-sections in the form of lobes,such as trilobes. The objects can be solid or hollow. They can have theform of a monolith, honeycomb or cylinder, for example.

[0013] The compositions of the invention are based on cerium oxide or oncerium and zirconium oxides. This is understood to mean that the ceriumoxide or the cerium oxide in combination with the zirconium oxiderepresent at least 50% by weight of the entire composition. They can becomposed essentially or solely of cerium and zirconium oxides with, inaddition, if appropriate, one or more additives of the type which willbe described hereinbelow.

[0014] In the case of compositions based on cerium and zirconium oxides,the respective proportions of cerium and of zirconium can vary withinwide limits. More particularly, this proportion, expressed by the Zr/Ceatomic ratio, can vary between 1/20 and 20/1, more particularly between1/9 and 9/1.

[0015] The compositions of the invention can comprise, in addition tocerium and zirconium, additives. These additives will be chosen fromthose known for improving the catalytic properties of cerium or ofzirconium. Use may thus be made of additives for stabilizing thespecific surface of these compositions or those known for increasingtheir oxygen storage capacity.

[0016] Mention may be made, as additives, of those belonging to thegroup consisting of aluminium, silicon, thorium, titanium, niobium,tantalum and rare-earth metals.

[0017] Rare-earth metal is understood to mean the elements of the groupconsisting of yttrium and elements of the Periodic Classification withan atomic number of between 57 and 71 inclusive. Mention may moreparticularly be made, among rare-earth metals, of yttrium, lanthanum,neodymium and praseodymium.

[0018] Mention may also be made, by way of additives, of those belongingto the group consisting of magnesium, scandium, hafnium, gallium andboron.

[0019] Finally, mention may be made of additives belonging to the groupconsisting of iron, bismuth, nickel, manganese, tin and chromium.

[0020] It is obvious that all the additives cited here can be present inthe compositions of the invention alone or in combination, whatever thegroup to which they belong. In addition, these additives are generallypresent in the compositions in the form of oxides.

[0021] The amounts of additives can vary within wide proportions. Themaximum amount is at most 50%, expressed as weight of oxide of additivewith respect to the weight of the entire composition. The minimum amountis that necessary in order to obtain the desired effect. Generally, thisamount is at least 0.1%. More particularly, the amount of additive canbe between 1 and 20% and more particularly still between 1 and 10%.

[0022] The compositions of the invention can exhibit high specificsurfaces, even after calcination at high temperature. This surfacedepends on the nature of the constituents of the composition. Thehighest surfaces will be obtained for the compositions in whichzirconium predominates.

[0023] More specifically, the compositions of the invention in whichcerium is present with zirconium and where cerium predominates, that isto say that the Ce/Zr atomic ratio is greater than 1, can exhibitspecific surfaces of at least 20 m²/g, more particularly of at least 30m²/g, after calcination at 900° C. for 6 hours. With one or moreadditives, especially such as scandium and rare-earth metals and inparticular lanthanum, praseodymium or neodymium, these surfaces can beat least 35 m²/g or alternatively at least 40 m²/g and more particularlystill at least 45 m²/g after calcination under the same conditions.These compositions with additives can also exhibit a surface of at least20 m²/g and more particularly still of at least 30 m²/g aftercalcination at 1000° C. for 6 hours.

[0024] Moreover, the compositions of the invention in which cerium ispresent with zirconium but where zirconium predominates, that is to saythat the Ce/Zr atomic ratio is less than 1, can exhibit specificsurfaces of at least 30 m²/g, more particularly of at least 40 m²/g,after calcination at 900° C. for 6 hours. With one or more additives,especially rare-earth metals and in particular lanthanum, praseodymiumor neodymium, these surfaces can be at least 50 m²/g or alternatively atleast 55 m²/g and more particularly still at least 60 m²/g aftercalcination under the same conditions. These compositions with additivescan also exhibit a surface of at least 30 m²/g and more particularlystill of at least 40 m²/g after calcination at 1000° C. for 6 hours.

[0025] Specific surface is understood to mean the B.E.T. specificsurface determined by nitrogen adsorption in accordance with ASTMStandard D 3663-78 based on the Brunauer-Emmett-Teller method describedin the periodical “The Journal of the American Chemical Society, 60, 309(1938)”.

[0026] The process for the preparation of the extruded compositions ofthe invention will now be described.

[0027] The main characteristic of the process of the invention is tostart from a specific product. This product can be defined in two ways.

[0028] The product which is subjected to extrusion may first of all bedefined as being a product based on a cerium hydroxide or oxyhydroxideor on cerium and zirconium hydroxides or oxyhydroxides. Such a hydroxidecan generally be represented by the formula (1) M(OH)_(x)(X)_(y).nH₂O,in which M represents cerium or zirconium and X an anion, x+y being atmost equal to 4 and x being other than 0, it being possible for y and nto be zero. The anion X is the anion of the cerium or zirconiumcompound, in particular a salt, which is generally used in thepreparation of the hydroxide, as will be described hereinbelow. It isalso possible to start from a product based on an oxyhydroxide offormula (2) MO_(z)(OH)_(x)(X)_(y).nH₂O, in which M and X have the samemeaning as above and where x+y+z is at most equal to 4, x and z beingother than 0 and it being possible for y and n to be zero. Such anoxyhydroxide can be obtained by drying a hydroxide of formula (1).

[0029] It should be noted that, in the formulae (1) and (2), therespective values of x, y, z and n can vary according in particular tothe preparation processes used to obtain the hydroxides oroxyhydroxides. Thus, by way of example, n can vary between approximately0 and 20 and y can be at most 0.5, X being in particular a nitrateanion. It is emphasized here that these values are not limiting, theessential characteristic of the invention being the use in the extrusionoperation of a product containing the hydroxide anion.

[0030] The product which is subjected to extrusion may also be definedby its preparation processes.

[0031] For the preparation of such a product, reference may be made inparticular to Patent Application EP-A-300,852 on behalf of the ApplicantCompany, the teaching of which is incorporated here, which describes thepreparation of a ceric hydroxide by reacting a cerium salt solution anda base, optionally in the presence of an oxidizing agent, the proportionof base being such that the pH of the reaction mixture is greater than7. The cerium hydroxide thus obtained can subsequently be subjected to ahot solvent treatment, in which it is resuspended in water or in adecomposable base and is heated in a closed chamber to a temperature anda pressure which are respectively lower than the critical temperatureand the critical pressure of the reaction mixture. Cerium(IV) hydroxidescan be obtained by hydrolysis of an aqueous cerium(IV) solution inacidic medium (Patent Application FR-A-2,596,380 on behalf of theApplicant Company).

[0032] As regards the preparation of cerium and zirconium hydroxides oroxyhydroxides or of products based on cerium and zirconium which aresuitable for extrusion in the context of the present invention,reference may be made to Patent Applications FR-A-2,699,524 andFR-A-2,714,370 on behalf of the Applicant Company, the teaching of whichis incorporated here, which describe in particular processes which makeit possible to obtain, on conclusion of some of their stages, productsof this type.

[0033] Thus, a first process of this type which can be describedcontains the following stages. A liquid mixture is formed comprisingcerium and zirconium compounds and, if appropriate, at least onecompound of an additive; the mixture obtained is heated; the precipitateformed is recovered; and the said precipitate is optionally dried. Theproduct thus obtained on conclusion of these stages is suitable forextrusion in the present invention.

[0034] The first stage of this process thus consists in preparing aliquid mixture, generally an aqueous mixture, containing cerium andzirconium compounds and, if appropriate, at least one compound of anadditive. These compounds are generally salts of the abovementionedelements and preferably soluble salts. The liquid mixture can beobtained without discrimination either from compounds initially in thesolid state, which will subsequently be introduced into a vessel waterheel for example, or alternatively directly from solutions of thesecompounds and then mixing, in any order, the said solutions.

[0035] Mention may in particular be made, as cerium compounds, of ceriumsalts such as cerium(IV) salts, such as nitrates or ceric ammoniumnitrates, for example, which are particularly well suited in thisinstance. Ceric nitrate is preferably used. The solution of cerium(IV)salts can contain cerium in the cerous state but it is preferable for itto contain at least 85% of cerium(IV). An aqueous ceric nitrate solutioncan, for example, be obtained by reaction of nitric acid with a cericoxide hydrate prepared conventionally by reaction of a solution of acerous salt, for example cerous nitrate, and of an aqueous ammoniasolution in the presence of aqueous hydrogen peroxide solution. Use mayalso be made of a ceric nitrate solution obtained according to theprocess for electrolytic oxidation of a cerous nitrate solution, asdescribed in the document FR-A-2,570,087, which can constitute anadvantageous starting material.

[0036] The zirconium compounds can be chosen from zirconium sulphate,zirconyl nitrate or zirconyl chloride. Zirconyl nitrate is particularlywell suited. Mention may more particularly be made of the use of azirconyl nitrate originating from the attack of nitric acid on zirconiumcarbonate. The zirconium compound can also be a salt of an organic acid,such as acetic acid or citric acid.

[0037] It should be noted here that the aqueous solutions of cerium(IV)salts and of zirconyl salts can exhibit a degree of initial freeacidity. According to the present invention, it is just as possible touse an initial solution of cerium(IV) and zirconium salts effectivelyexhibiting a degree of free acidity as mentioned above as solutionswhich would have been neutralized beforehand more or less exhaustively.This neutralization can be carried out by addition of a basic compoundto the abovementioned mixture, so as to limit this acidity. This basiccompound can be, for example, an aqueous ammonia solution oralternatively a solution of alkali metal (sodium, potassium and thelike) hydroxides but preferably an aqueous ammonia solution. It is thenpossible to define in practice a degree of neutralization (r) of theinitial cerium and zirconium solution by the following equation:$r = \frac{{n\quad 3} - {n\quad 2}}{n\quad 1}$

[0038] in which n1 represents the total number of moles of Ce(IV) and ofzirconium present in the solution after neutralization; n2 representsthe number of moles of OH⁻ ions effectively necessary to neutralize theinitial free acidity introduced by the aqueous cerium(IV) and zirconiumsalt solutions; and n3 represents the total number of moles of OH⁻ ionsintroduced by the addition of the base. When the “neutralization”alternative form is implemented, use is made in all cases of an amountof base which absolutely must be less than the amount of base whichwould be necessary to obtain complete precipitation of the ceriumzirconium hydroxide species, this amount depending on the compositionsynthesized. In practice, the limit is therefore set at degrees ofneutralization which do not exceed 2.

[0039] According to a specific embodiment, use is made, as zirconiumcompound, of a zirconium solution which exhibits the followingcharacteristic. The amount of base necessary to reach the equivalentpoint during acid/base quantitative determination of this solution mustmeet the condition OH⁻/Zr molar ratio ≦1.65. More particularly, thisratio can be at most 1.5 and more particularly still at most 1.3.

[0040] Acid/base quantitative determination is carried out in a knownway. To carry it out under optimum conditions, a solution which has beenbrought to a concentration of approximately 3×10⁻² mol per liter,expressed as elemental zirconium, can be quantitatively determined. A 1Nsodium hydroxide solution is added thereto with stirring. Under theseconditions, the equivalent point (change in the pH of the solution) isdetermined sharply. This equivalent point is expressed by the OH⁻/Zrmolar ratio.

[0041] Mention may be made, as compounds of the additives which can beused in the process of the invention, of, for example, salts ofinorganic or organic acids, for example of the sulphate, nitrate,chloride or acetate type. It should be noted that the nitrate isparticularly well suited. These compounds can also be introduced in theform of sols. These sols can be obtained, for example, by neutralizationby a base of a salt of these compounds.

[0042] The amounts of cerium, of zirconium and optionally of additivespresent in the mixture must correspond to the stoichiometric proportionsrequired in order to obtain the desired final composition.

[0043] The initial liquid mixture being thus obtained, it issubsequently heated in accordance with the second stage of the process.

[0044] The temperature at which this heat treatment, also known asthermal hydrolysis, is carried out can be between 80° C. and thecritical temperature of the reaction mixture, in particular between 80and 350° C., preferably between 90 and 200° C.

[0045] This treatment can be carried out, according to the temperatureconditions employed, either at normal atmospheric pressure or underpressure, such as, for example, the saturated vapour pressurecorresponding to the temperature of the heat treatment. When thetreatment temperature is chosen greater than the reflux temperature ofthe reaction mixture (that is to say, generally greater than 100° C.),for example chosen between 150 and 350° C., the operation is thencarried out by introducing the aqueous mixture containing theabovementioned species into a closed chamber (closed reactor, morecommonly known as an autoclave), the necessary pressure then resultingonly from the heating alone of the reaction mixture (autogenouspressure). Under the temperature conditions given above, and in aqueousmedia, it is thus possible to specify, by way of illustration, that thepressure in the closed reactor varies between a value greater than 1 bar(10⁵ Pa) and 165 bar (165×10⁵ Pa), preferably between 5 bar (5×10⁵ Pa)and 165 bar (165×10⁵ Pa). It is, of course, also possible to exert anexternal pressure which is then added to that resulting from theheating.

[0046] The heating can be carried out either under an air atmosphere orunder an inert gas atmosphere, preferably nitrogen.

[0047] The duration of the treatment is not critical and can thus varywithin wide limits, for example between 1 and 48 hours, preferablybetween 2 and 24 hours.

[0048] On conclusion of the heating stage, a solid precipitate isrecovered which can be separated from its mixture by any conventionalsolid/liquid separation technique, such as, for example, filtration,settling, draining or centrifuging.

[0049] It may be advantageous, on conclusion of this second stage, tobring the reaction mixture thus obtained to a basic pH. This operationis carried out by adding a base, such as, for example, an aqueousammonia solution, to the mixture.

[0050] Basic pH is understood to mean a pH value greater than 7 andpreferably greater than 8.

[0051] It is also possible to add, in the same way, after the heatingstage, aqueous hydrogen peroxide solution.

[0052] It should be noted that it is, of course, possible to repeat, oneor a number of times, in an identical or nonidentical way, afterrecovery of the product and possible addition of the base or of theaqueous hydrogen peroxide solution, a heating stage as described above,the product then again being placed in liquid medium, in particular inwater, and heat treatment cycles, for example, being carried out.

[0053] The product as recovered can subsequently be subjected to washingoperations with water and/or with aqueous ammonia, at a temperature ofbetween room temperature and the boiling temperature. In order to removethe residual water, the washed product can finally, optionally, bedried, for example with air, at a temperature which can vary between 80and 300° C., preferably between 100 and 150° C., drying being continueduntil a constant weight is obtained.

[0054] It is also possible to carry out drying by atomization of theprecipitate obtained. In this case, it is then unnecessary to separatethe precipitate from the reaction mixture in which it has been obtained.The reaction mixture can be left to separate by settling, thesupernatant can then be drawn off and, finally, drying can be carriedout by atomization.

[0055] A second process for the preparation of the cerium and zirconiumhydroxides or oxyhydroxides can also be employed in which aprecipitation is carried out. According to this second process, a liquidmixture is formed comprising a cerium compound or cerium and zirconiumcompounds and, if appropriate, at least one compound of an additive; abase is added to the mixture obtained; the precipitate formed isrecovered; and the said precipitate is optionally dried. The productthus obtained on conclusion of these stages is suitable for extrusion inthe present invention.

[0056] That which has been described above with respect to the compoundsof cerium, of zirconium and of the additives also applies here.

[0057] In the second stage of this second process, a base is added tothe liquid mixture formed previously. Products of the hydroxide type canbe used as base. Alkali metal or alkaline-earth metal hydroxides may bementioned. Secondary, tertiary or quaternary amines can also be used.However, amines and ammonia may be preferred insofar as they decreasethe risks of pollution by alkali metal or alkaline-earth metal cations.Mention may also be made of urea. The reactants can be introduced in anyorder, it being possible for the base to be introduced into the mixtureor vice versa or it alternatively being possible for the reactants to beintroduced simultaneously into the reactor.

[0058] Addition can be carried out on a single occasion, gradually orcontinuously, and it is preferably carried out with stirring. Thisoperation can be carried out at a temperature of between roomtemperature (18-25° C.) and the reflux temperature of the reactionmixture, it being possible for the latter to reach 120° C., for example.It is preferably carried out at room temperature.

[0059] At the end of the addition of the base, the reaction mixture canoptionally still be kept stirring for some time, in order to bring theprecipitation to completion.

[0060] When the base is added continuously, the pH of the reactionmixture is preferably maintained between approximately 7 andapproximately 11, more particularly between 7.5 and 9.5.

[0061] An alternative form of this second process can be stated here,this alternative form corresponding to the teaching of FR-A-2,714,370,which has been mentioned above. In this alternative form, the reactionwith the base takes place in the presence of a carbonate or bicarbonate.The term carbonate must be understood as also being able to comprise ahydroxycarbonate. Mention will be made, by way of example, of ammoniumcarbonate or bicarbonate. Moreover, the reaction takes place underconditions such that the pH of the reaction mixture remains neutral orbasic. The pH value of the reaction mixture is generally at least 7 andis between 7 and 7.5 in the case of a neutral mixture and moreparticularly at least 8 in the case of a basic mixture. Morespecifically, this value can be between 7.5 and 14, in particularbetween 8 and 11 and more particularly between 8 and 9.

[0062] According to a specific embodiment of this alternative form, theliquid mixture comprising the cerium and zirconium compounds andoptionally the compound of the additive is introduced, with thecarbonate or the bicarbonate, into a basic solution. It is thuspossible, for example, to form a vessel heel with the basic solution,into which the liquid mixture is introduced.

[0063] It is also possible, in this alternative form, to carry out theoperation continuously. In this case, the liquid mixture comprising thecerium and zirconium compounds and optionally the compound of theadditive, the base and the carbonate or the bicarbonate are introducedsimultaneously into a reactor, an excess of base being provided in orderto fulfil the pH condition.

[0064] On conclusion of the precipitation stage of the second process, amass of a solid precipitate is recovered which can be separated from itsmixture and optionally dried as described above for the first process.

[0065] The cerium and zirconium hydroxide(s) or oxyhydroxide(s) and theproduct(s) which have been obtained by the processes described abovewill subsequently be shaped by extrusion.

[0066] They can be extruded directly or else in the form of a mixturewith an acidic solution. The presence of an acidic solution facilitatesthe shaping. Use may be made, as acid, of, for example, nitric acid,stearic acid, oxalic acid or acetic acid. The amount of acid used isgenerally between approximately 0.1 and 5%, expressed as moles of acidwith respect to the sum of the moles of cerium and of zirconium.

[0067] The cerium and zirconium hydroxide(s) or oxyhydroxide(s) and theproduct(s) which have been obtained by the processes described above canalso be extruded as a mixture with known shaping additives, such ascellulose, carboxymethylcellulose, carboxyethylcellulose, xanthan andsuccinoglycan gums, surface-active agents, flocculating agents, such aspolyacrylamides, carbon black, starches, poly(acrylic alcohol),poly(vinyl alcohol), glucose or polyethylene glycols.

[0068] An important advantage of the invention is that the product to beextruded can be extruded in the absence of the binders which arenormally used in this type of technique. Of course, it would not bedeparting from the scope of the present invention to use a binder whichcan be chosen from silica, alumina, clays, silicates, titanium sulphateor ceramic fibres, in particular in proportions normally used, that isto say up to approximately 30% by weight.

[0069] The cerium and zirconium hydroxide(s) or oxyhydroxide(s) and theproduct(s) which have been obtained by the processes described above,either alone or as a mixture with the abovementioned acidic solution orshaping additives or alternatively the abovementioned binders,preferably exhibit, before extrusion, a loss on ignition of between 25and 75%, more particularly between 40 and 65%. The loss on ignition(LOI) is measured as the loss in weight corresponding to the ratio:

LOI in %=(W₀−W₁)/W₀, in which:

[0070] W₀ is the initial weight of the starting material

[0071] W₁ is the weight of this starting material after calcination for2 hours at 1000° C. and cooling to room temperature in an anhydrouschamber.

[0072] Prior to the extrusion, the cerium and zirconium hydroxide(s) oroxyhydroxide(s) and the product(s) which have been obtained by theprocesses described above are kneaded. The duration of the kneading canvary within wide limits, for example between 1 minute and three hours.

[0073] Extrusion takes place in any appropriate device.

[0074] The extruded product is optionally dried and subsequentlycalcined.

[0075] Calcination is carried out at a temperature generally of between200 and 1200° C., preferably between 300 and 900° C. and moreparticularly still between 500° C. and 900° C. This calcinationtemperature must be sufficient to convert the precursors to oxides andit is also chosen as a function of the temperature of subsequent use ofthe catalytic composition and by taking into account the fact that thespecific surface of the product decreases as the calcination temperatureemployed increases. The duration of the calcination can, for its part,vary within wide limits, for example between 1 and 24 hours, preferablybetween 4 and 10 hours. The calcination is generally carried out underair but a calcination carried out, for example, under inert gas is veryclearly not excluded.

[0076] The extrudates thus obtained can be employed in combination withcatalytically active metals of the precious metal type. The nature ofthese metals and the techniques for the incorporation of the latter inthese compositions are well known to a person skilled in the art. Forexample, the metals can be platinum, rhodium, palladium, ruthenium oriridium; they can in particular be incorporated in the compositions byimpregnation.

[0077] The extrudates of the invention can be used very particularly inthe treatment of exhaust gases from internal combustion engines, in theprocess for the dehydrogenation of ethylbenzene to styrene or in thecatalysis of methanation.

[0078] More generally, they can be used in the catalysis of variousreactions, such as, for example, dehydration, hydrosulphurization,hydrodenitrification, desulphurization, hydrodesulphurization,dehydrohalogenation, reforming, steam reforming, cracking,hydrocracking, hydrogenation, dehydrogenation, isomerization,dismutation, oxychlorination, dehydrocyclization of hydrocarbons or ofother organic compounds, oxidation and/or reduction reactions, the Clausreaction, demetallation, the shift conversion or the treatment of asolution or suspension of organic compounds by oxidation via a wetroute.

[0079] The latter treatment of an aqueous solution or suspension oforganic compounds takes place at a high temperature and a high pressureby oxidation of the organic compounds by gas containing oxygen in thepresence of an oxidation catalyst in order to reduce the chemical oxygendemand of the said solution or suspension to a predetermined level. Itis characterized in that the catalyst comprises a catalytically activephase present on a support composed of a composition based on a ceriumoxide and on a zirconium oxide in a cerium/zirconium atomic proportionof at least 1, exhibiting a specific surface, after calcination for 6hours at 900° C., of at least 35 m²/g and an oxygen storage capacity at400° C. of at least 1.5 ml of O₂/g. This support can be provided in theform of an extrudate. The catalytically active phase is composed ofruthenium or of iridium in the metal form or in the oxide form.

[0080] The oxidation reaction is carried out by employing, as oxidizinggas, a gas containing oxygen, such as, for example, pure oxygen, air,air enriched with oxygen or residue gases containing oxygen.

[0081] The amount of gas supplied is determined from the chemical oxygendemand (COD) of the solution to be treated. Generally, the gascontaining oxygen is used in an amount equal to 1 to 1.5 times thetheoretical amount of oxygen.

[0082] The oxygen pressure is advantageously between 1 and 50 bar, thetotal pressure of the gases being sufficiently high to keep the solutionor suspension in the liquid state at the temperature of the reaction.

[0083] This reaction temperature is advantageously between 100° C. and400° C., preferably between 120° C. and 200° C. This temperature dependsin particular on the nature of the organic compounds present in theeffluents to be treated.

[0084] The aqueous solutions or suspensions which can be treated by thisprocess are aqueous liquors which preferably contain oxidizable organicsubstances, such as aqueous effluents exhibiting a moderatelyconcentrated chemical oxygen demand, advantageously of less than 200g/l.

[0085] Examples of waste water are, for example, waste water originatingfrom industrial plants, such as the chemical or oil industries,municipal effluents, waste water containing oils or waste wateroriginating from a gas purification process or an activated sludgeprocess. Advantageously, to avoid fouling the plants and the catalyst,these aqueous liquors can be filtered before being treated.

[0086] Examples will now be given.

EXAMPLE 1

[0087] This example illustrates the preparation of an oxide support inthe extruded form of formula Ce_(0.62)Zr_(0.38)O₂ .

[0088] A ceric nitrate solution and a zirconyl nitrate solution aremixed in the stoichiometric proportions required in order to obtain theabove mixed oxide. The zirconyl nitrate solution was obtained by attackon a carbonate using concentrated nitric acid. The solution meets,within the meaning defined above, the condition OH⁻/Zr molar ratio=0.94.

[0089] The concentration of this mixture (expressed as oxides of thevarious elements) is adjusted to 80 g/l. This mixture is subsequentlybrought to 150° C. for 4 hours.

[0090] An aqueous ammonia solution is subsequently added to the reactionmixture, so that the pH is greater than 8.5. The reaction mixture thusobtained is brought to boiling point for 2 hours. After separation bysettling and then drawing off, the solid product is resuspended and themixture thus obtained is treated for 1 hour at 100° C. The product issubsequently filtered off. The filtration cake thus obtained exhibits aloss on ignition at 1000° C. of 60.6%. The cerium/zirconium hydroxide oroxyhydroxide is kneaded for 15 minutes before being shaped by extrusionthrough a 3.2 mm die. The equipment used is a single-screw extruder soldby the company Lhomargie. The extrudates obtained have the form of acylinder with a diameter of 1.2 mm. They are subsequently dried at 100°C. for 1 hour before being calcined at 600° C. under air.

[0091] The shaped oxide exhibits a specific surface, after treatment at900° C. under air for 6 hours, of 39 m²/g and, at 1000° C. for 6 hours,of 17 m²/g.

EXAMPLE 2

[0092] This example illustrates the preparation of an oxide support inthe extruded form of formula Ce_(0.17)Zr_(0.83)O₂.

[0093] A zirconyl nitrate solution (obtained by attack of nitric acid ona Zr carbonate) is added to a cerium(IV) nitrate solution in therespective proportions, by weight of oxide, of 80/20 and such that theratio r as defined above is 0.5. The concentration is adjusted to 80 g/land the solution is then brought to 150° C. for 6 hours. After cooling,the pH of the reaction mixture is brought to a value of 8.5 using anammonia solution. The temperature is subsequently brought to 100° C.After cooling, the mother liquors are removed by settling and anequivalent amount of water is added. The reaction mixture is againbrought to 100° C. After separation by settling, the supernatant isremoved and the product is dried by atomization. The powder obtained iskneaded with an aqueous nitric acid solution with a concentration suchthat the HNO₃/Zr+Ce ratio is equal to 0.025 and the loss on ignition ofthe paste at 1000° C. is 45%. The cerium/zirconium hydroxide obtained issubsequently shaped as in Example 1 and then dried at 100° C. for 1 hourbefore being calcined at 500° C. under air.

[0094] The specific surface of the oxide thus obtained is 45 m²/g aftercalcination for 6 hours at 900° C.

[0095] X-ray diffraction analysis shows that the oxide obtained is inthe form of a solid solution pure phase.

EXAMPLE 3

[0096] This example illustrates the preparation of an oxide support inthe extruded form of formula Ce_(0.75)Zr_(0.25)O₂.

[0097] The two following solutions constitute the starting materials.Solution 1: 116.7 g of Ce(III) nitrate (29.5% CeO₂₎ 24.7 g of zirconylnitrate 27 (19.9% ZrO) 124 g of H₂O Solution 2: 47.4 g of ammoniumbicarbonate 35.2 g of NH₄OH (29% as NH₃) 240.4 g of H₂O

[0098] Solution 1 is preheated to 80° C. It is added over 5 minutes toSolution 2, originally maintained at a temperature of 25° C. The mixtureis filtered through a Büchner funnel with a diameter of 15 cm and theresidue is washed with 500 ml of water. The product is subsequentlydried at 125° C. in a ventilated oven.

[0099] The powder obtained is shaped as in Example 1 with an addition ofan aqueous acetic acid solution with a concentration such that theCH₃CO₂H/Zr+Ce ratio is equal to 0.030 and the loss on ignition of thepaste at 1000° C. is 51%.

[0100] The cerium/zirconium hydroxide obtained is dried at 100° C. for 1hour before being calcined at 700° C. under air.

[0101] The shaped oxide exhibits a specific surface, after treatment at900° C. under air for 6 hours, of 30 m²/g.

1. Composition based on cerium oxide or on cerium and zirconium oxides,characterized in that it is provided in the extruded form. 2.Composition according to claim 1, characterized in that it is based oncerium and zirconium oxides in a Zr/Ce atomic ratio of between 1/20 and20/1, more particularly between 1/9 and 9/1.
 3. Composition according toclaim 1 or 2, characterized in that it additionally comprises at leastone additive chosen from the group consisting of aluminium, silicon,thorium, titanium, niobium, tantalum and rare-earth metals. 4.Composition according to one of the preceding claims, characterized inthat it additionally comprises at least one additive chosen from thegroup consisting of magnesium, scandium, hafnium, gallium and boron. 5.Composition according to one of the preceding claims, characterized inthat it additionally comprises at least one additive chosen from thegroup consisting of iron, bismuth, nickel, manganese, tin and chromium.6. Composition according to one of the preceding claims, characterizedin that it exhibits a specific surface of at least 20 m²/g, moreparticularly of at least 30 m²/g, after calcination at 900° C. for 6hours.
 7. Process for the preparation of a composition according to oneof the preceding claims, characterized in that a product which is basedon a cerium hydroxide or oxyhydroxide or on cerium and zirconiumhydroxides or oxyhydroxides is extruded.
 8. Process for the preparationof a composition according to one of claims 1 to 6, characterized inthat the extrusion is carried out of a product obtained by a process inwhich a liquid mixture comprising a cerium compound or cerium andzirconium compounds and, if appropriate, at least one compound of anadditive is formed; the mixture obtained is heated; the precipitateformed is recovered; and the said precipitate is optionally dried. 9.Process for the preparation of a composition according to one of claims1 to 6, characterized in that the extrusion is carried out of a productobtained by a process in which a liquid mixture comprising a ceriumcompound or cerium and zirconium compounds and, if appropriate, at leastone compound of an additive is formed; a base is added to the mixtureobtained; the precipitate formed is recovered; and the said precipitateis optionally dried.
 10. Process for the preparation of a compositionaccording to one of claims 1 to 6, characterized in that the extrusionis carried out of a product obtained by a process in which a cerium saltsolution and a base are reacted, optionally in the presence of anoxidizing agent, the proportion of base being such that the pH of thereaction mixture is greater than
 7. 11. Process for the preparation of acomposition according to one of claims 1 to 6, characterized in that theextrusion is carried out of a product obtained by a process in which anaqueous cerium(IV) solution is hydrolysed in acidic medium.
 12. Processaccording to one of claims 7 to 11, characterized in that the extrusionis carried out of a mixture obtained, on the one hand, from theabovementioned hydroxide(s) or oxyhydroxide(s) or at least one of theproducts obtained by the abovementioned processes and, on the otherhand, from an acidic solution.
 13. Process according to one of claims 7to 12, characterized in that the extrusion is carried out either of oneor more abovementioned hydroxide(s) or oxyhydroxide(s) or at least oneof the products obtained by the abovementioned processes or of a mixtureobtained from one or more abovementioned hydroxide(s) or oxyhydroxide(s)or from at least one of the products obtained by the abovementionedprocesses which exhibit a loss on ignition of between 25 and 75%, moreparticularly between 40 and 65%.
 14. Use of a composition according toone of claims 1 to 6 as catalyst or catalyst support, in particular inthe treatment of exhaust gases from internal combustion engines, in theprocess for the dehydrogenation of ethylbenzene to styrene, in thecatalysis of methanation or in the treatment of a solution or suspensionof organic compounds by oxidation via a wet route.